1. Field of the Invention:
This invention relates to viscous fluid coupling assemblies for use in automobiles in general, and more particularly, to an improvement in a viscous fluid coupling assembly which can control the transmission of the output torque in three steps.
2. Description of the Prior Art:
In a conventional viscous fluid coupling assembly for driving a fan assembly of automobiles, the viscous fluid communication from a reservoir chamber to a working chamber is controlled in response to changes in temperature in order to prevent the overcooling of the engine and to reduce the loss of horsepower. In the conventional assembly, however, a thermal responsive type valve member operates only to open and close a connecting hole between the reservoir chamber and the working chamber. Therefore, the transmission of the output torque can be controlled only in two steps in response to the opening and the closing of the connecting hole. Accordingly, in particular in a temperature range wherein the thermal responsive valve member operates so that the transmission of the output torque is changed from a first condition where a small output torque is transmitted to a second condition where a large output torque is transmitted, the desired cooling function can not be attained or the over-cooling function can be attained. As a result, various disadvantages, such as the increase of the noise and the loss of the horsepower will arise.
In order to obviate the various disadvantages mentioned above, the U.S. Pat. No. 4,298,111, granted Nov. 3, 1981, and assigned to the same assignee as that of the present invention, teaches a viscous fluid coupling assembly which can control the transmission of the output torque in three steps. In this U.S. Patent there is no problem when the transmission of the output torque is controlled in three steps beginning from the condition of non-operation. However, since the pump which brings the viscous fluid contained in the working chamber back to the reservoir chamber is operated by means of the relative rotation between the rotor and the casing, the pump is not operated until the relative rotation between the rotor and the casing increases, even if the opening which supplies the viscous fluid to one of the labyrinths is closed after the viscous fluid is supplied to both labyrinths which are formed in both sides of the rotor in the working chamber. On the other hand, the opening which supplies the viscous fluid to the other labyrinth is open. Since this opening is formed radially outwardly of the above-mentioned closed opening, a large volume of viscous fluid is supplied from the reservoir chamber to the working chamber. Therefore, the viscous fluid can't be pumped rapidly enough from the working chamber back to the reservoir chamber. As a result, a delay in the operation would occur.